Molten-droplet-driven growth of MoS2 flakes with controllable morphology transition for hydrogen evolution reactions.

The controllable fabrication of two-dimensional transition-metal dichalcogenides (2D TMDs) and a deep understanding of the corresponding process mechanisms are of fundamental importance for their further applications. In this study, the molten-droplet-driven (MDD) growth of MoS2 based on a Na-Mo-O molten-salt chemical vapor deposition (CVD) method is demonstrated via temperature-dependent dispersion and spreading of droplets on a surface, yielding MoS2 flakes with morphology transition from compact triangles to fractal dendrites with the increase in temperature. By building up the dependence between the formed morphologies of grown MoS2 flakes and the corresponding kinetics during successive growth processes, it was found that the wetting-driven force, which is governed by interface free energies (surface tension) of molten droplets, would largely determine the driven movement of the droplet, and then the formation of different morphologies. Finally, based on these MoS2 flakes, a systematic improvement of the hydrogen evolution reaction (HER) was demonstrated in accordance with the evolution of morphologies from compact to fractal. This study presents an important advance in understanding the growth mechanisms related to the molten-salt-assisted CVD fabrication of 2D TMDs and provides a facile method for tailoring the growth and application of 2D TMDs with controllably trimmed morphologies.

Chemical Vapor Deposition of N-Doped Graphene through Pre-Implantation of Nitrogen Ions for Long-Term Protection of Copper.

Nitrogen-doped graphene (NG) was synthesized through the chemical vapor deposition (CVD) of graphene on Cu substrates, which were pre-implanted with N ions by the ion implantation method. The pre-implanted N ions in the Cu substrate could dope graphene by the substitution of C atoms during the CVD growth of graphene, forming NG. Based on this, NG's long-term protection properties for Cu were evaluated by ambient exposure for a corrosion test. The results showed that NG can obviously reduce the natural oxidation of Cu in the long-term exposure compared with the case of pristine graphene (PG) coated on Cu. Moreover, with the increase in pre-implanted N dose, the formed NG's long-term protection for Cu improved. This indicates that the modification of graphene by N doping is an effective way to improve the corrosion resistance of the PG coating owing to the reduction in its conductivity, which would inhibit galvanic corrosion by cutting off electron transport across the interface in their long-term protection. These findings provide insight into corrosion mechanisms of the graphene coating and correlate with its conductive nature based on heteroatoms doping, which is a potential route for improving the corrosion resistance of graphene as an effective barrier coating for metals.

Selective area oxidation of copper derived from chemical vapor deposited graphene microstructure.

The barrier properties of graphene coating are highly correlated with its microstructure which is then determined by the chemical vapor deposition (CVD) growth history on metals. We demonstrate here an unrevealed selective area oxidation of copper under graphene, which is derived from the implicit-etching-controlled CVD growth mode of graphene. By charactering and analyzing the selective area patterns of Cu oxidation, an etched pattern trace with nano/microvoids during graphene growth has been proposed to account for this. Based on such selective oxidation of Cu, distributed galvanic corrosion will be triggered and proceed locally at the interface of graphene-Cu system to coalescence together under a continuous corrosion environment, eventually presenting a homogeneous oxidation of Cu and gradual decoupling of graphene-Cu system. This discovery will assist our understanding of the barrier properties of two-dimensional materials and can be extended to other applications related to quality monitoring of grown materials and defects-based chemical modifications.

Graphene-Subgrain-Defined Oxidation of Copper.

The correlation between the crystal structure of chemical vapor deposition (CVD)-grown graphene and the crystal structure of the Cu growth substrate and their mutual effect on the oxidation of the underlying Cu are systematically explored. We report that natural oxygen or water intercalation along the graphene-Cu interface results in an orientation-dependent oxidation rate of the Cu surface, particularly noticeable for bicrystal graphene domains on the same copper grain, suggesting that the relative crystal orientation of subgrains determines the degree of Cu oxidation. Atomistic force field calculations support these observations, showing that graphene domains have preferential alignment with the Cu(111) with a smaller average height above the global Cu surface as compared to intermediate orientations, and that this is the origin of the heterogeneous oxidation rate of Cu. This work demonstrates that the natural oxidation resistance of Cu coated by graphene is highly dependent on the crystal orientation and lattice alignment of Cu and graphene, which is key information for engineering the interface configuration of the graphene-Cu system for specific functionalities in mechanical, anticorrosion, and electrical applications of CVD-grown graphene.

Graphene-Si CMOS oscillators.

Graphene field-effect transistors (GFETs) offer a possibility of exploiting unique physical properties of graphene in realizing novel electronic circuits. However, graphene circuits often lack the voltage swing and switchability of Si complementary metal-oxide-semiconductor (CMOS) circuits, which are the main building block of modern electronics. Here we introduce graphene in Si CMOS circuits to exploit favorable electronic properties of both technologies and realize a new class of simple oscillators using only a GFET, Si CMOS D latch, and timing RC circuit. The operation of the two types of realized oscillators is based on the ambipolarity of graphene, i.e., the symmetry of the transfer curve of GFETs around the Dirac point. The ambipolarity of graphene also allowed to turn the oscillators into pulse-width modulators (with a duty cycle ratio ∼1 : 4) and voltage-controlled oscillators (with a frequency ratio ∼1 : 8) without any circuit modifications. The oscillation frequency was in the range from 4 kHz to 4 MHz and limited only by the external circuit connections, rather than components themselves. The demonstrated graphene-Si CMOS hybrid circuits pave the way to the more widespread adoption of graphene in electronics.

Broadband, electrically tunable third-harmonic generation in graphene.

Optical harmonic generation occurs when high intensity light (>1010 W m-2) interacts with a nonlinear material. Electrical control of the nonlinear optical response enables applications such as gate-tunable switches and frequency converters. Graphene displays exceptionally strong light-matter interaction and electrically and broadband tunable third-order nonlinear susceptibility. Here, we show that the third-harmonic generation efficiency in graphene can be increased by almost two orders of magnitude by controlling the Fermi energy and the incident photon energy. This enhancement is due to logarithmic resonances in the imaginary part of the nonlinear conductivity arising from resonant multiphoton transitions. Thanks to the linear dispersion of the massless Dirac fermions, gate controllable third-harmonic enhancement can be achieved over an ultrabroad bandwidth, paving the way for electrically tunable broadband frequency converters for applications in optical communications and signal processing.

Quantitative optical mapping of two-dimensional materials.

The pace of two-dimensional materials (2DM) research has been greatly accelerated by the ability to identify exfoliated thicknesses down to a monolayer from their optical contrast. Since this process requires time-consuming and error-prone manual assignment to avoid false-positives from image features with similar contrast, efforts towards fast and reliable automated assignments schemes is essential. We show that by modelling the expected 2DM contrast in digitally captured images, we can automatically identify candidate regions of 2DM. More importantly, we show a computationally-light machine vision strategy for eliminating false-positives from this set of 2DM candidates through the combined use of binary thresholding, opening and closing filters, and shape-analysis from edge detection. Calculation of data pyramids for arbitrarily high-resolution optical coverage maps of two-dimensional materials produced in this way allows the real-time presentation and processing of this image data in a zoomable interface, enabling large datasets to be explored and analysed with ease. The result is that a standard optical microscope with CCD camera can be used as an analysis tool able to accurately determine the coverage, residue/contamination concentration, and layer number for a wide range of presented 2DMs.

Probing the Gas-Phase Dynamics of Graphene Chemical Vapour Deposition using in-situ UV Absorption Spectroscopy.

The processes governing multilayer nucleation in the chemical vapour deposition (CVD) of graphene are important for obtaining high-quality monolayer sheets, but remain poorly understood. Here we show that higher-order carbon species in the gas-phase play a major role in multilayer nucleation, through the use of in-situ ultraviolet (UV) absorption spectroscopy. These species are the volatilized products of reactions between hydrogen and carbon contaminants that have backstreamed into the reaction chamber from downstream system components. Consequently, we observe a dramatic suppression of multilayer nucleation when backstreaming is suppressed. These results point to an important and previously undescribed mechanism for multilayer nucleation, wherein higher-order gas-phase carbon species play an integral role. Our work highlights the importance of gas-phase dynamics in understanding the overall mechanism of graphene growth.

[Simultaneous determination of 13 organic pollutants in water samples by high performance liquid chromatography-tandem mass spectrometry].

On the basis of environmental quality standards for surface water, a sensitive and simple method was developed for the simultaneous determination of 13 organic pollutants, including dimethoate, dichlorvos, trichlorfon, parathion, methyl parathion, malathion, demeton, acrylamide, aniline, benzidine, carbaryl, Microcystin-LR and atrazine in water samples by high performance liquid chromatography-tandem mass spectrometry (HPLC-MS/MS) combined with direct injection. The water samples were filtered through 0.22 µm of microfiltration membrane. With methanol-0.01% formic acid as mobile phase, the separation was performed on a Kromasil 100-5 C18 column (150 mm x 2.1 mm, 5 µm) in gradient elution. The flow rate was 0.5 mL/min. The column temperature was 40 °C. The samples were detected by multiple reaction monitoring (MRM) mode with positive electro spray ionization. The organic pollutants were quantified by external standard method. The calibration curves of the organic pollutants show good linearity in suitable ranges with correlation coefficients (r) over 0.9995. The detection limits of organic pollutants in samples ranged from 0.02 µg/L to 0.1 µg/L. The relative standard deviations of organic pollutants were 0.5%-5.0% (n= 6). The average recoveries of the 13 organic pollutants spiked in water samples ranged from 81.2% to 112%. The method has been proven to be sensitive, rapid and with little interference. It is suitable for the determination of the 13 organic pollutants simultaneously in the surface and underground waters.

Controlled growth of single-crystal twelve-pointed graphene grains on a liquid Cu surface.

The controlled fabrication of single-crystal twelve-pointed graphene grains is demonstrated for the first time by ambient pressure chemical vapor deposition on a liquid Cu surface. An edge-diffusion limited mechanism is proposed. The highly controllable growth of twelve-pointed graphene grains presents an intriguing case for the fundamental study of graphene growth and should exhibit wide applications in graphene-based electronics.

Layer-stacking growth and electrical transport of hierarchical graphene architectures.

Hierarchical graphene architectures (HGAs) that grow by stacking of layers are produced on a liquid copper surface using chemical vapor deposition. The stacking mode--for example hexagonal-hexagonal-hexagonal or hexagonal-snowflake-dendritic--can be simply controlled. Measurements of the electrical properties of HGAs indicate that hierarchical stacking of graphene may be a simple and effective way of tailoring their properties without degrading them.

Fractal etching of graphene.

An anisotropic etching mode is commonly known for perfect crystalline materials, generally leading to simple Euclidean geometric patterns. This principle has also proved to apply to the etching of the thinnest crystalline material, graphene, resulting in hexagonal holes with zigzag edge structures. Here we demonstrate for the first time that the graphene etching mode can deviate significantly from simple anisotropic etching. Using an as-grown graphene film on a liquid copper surface as a model system, we show that the etched graphene pattern can be modulated from a simple hexagonal pattern to complex fractal geometric patterns with sixfold symmetry by varying the Ar/H2 flow rate ratio. The etched fractal patterns are formed by the repeated construction of a basic identical motif, and the physical origin of the pattern formation is consistent with a diffusion-controlled process. The fractal etching mode of graphene presents an intriguing case for the fundamental study of material etching.

Two-stage metal-catalyst-free growth of high-quality polycrystalline graphene films on silicon nitride substrates.

By using two-stage, metal-catalyst-free chemical vapor deposition (CVD), it is demonstrated that high-quality polycrystalline graphene films can directly grow on silicon nitride substrates. The carrier mobility can reach about 1500 cm(2) V(-1) s(-1) , which is about three times the value of those grown on SiO(2) /Si substrates, and also is better than some examples of metal-catalyzed graphene, reflecting the good quality of the graphene lattice.

Low temperature growth of highly nitrogen-doped single crystal graphene arrays by chemical vapor deposition.

The ability to dope graphene is highly important for modulating electrical properties of graphene. However, the current route for the synthesis of N-doped graphene by chemical vapor deposition (CVD) method mainly involves high growth temperature using ammonia gas or solid reagent melamine as nitrogen sources, leading to graphene with low doping level, polycrystalline nature, high defect density and low carrier mobility. Here, we demonstrate a self-assembly approach that allows the synthesis of single-layer, single crystal and highly nitrogen-doped graphene domain arrays by self-organization of pyridine molecules on Cu surface at temperature as low as 300 °C. These N-doped graphene domains have a dominated geometric structure of tetragonal-shape, reflecting the single crystal nature confirmed by electron-diffraction measurements. The electrical measurements of these graphene domains showed their high carrier mobility, high doping level, and reliable N-doped behavior in both air and vacuum.

[Differentiation of Rhododendron based on the infrared spectra of petals].

Several techniques were used to identify and classify plants. Mid-infrared spectroscopy combined with appropriate software was used in an attempt to differentiate different subgenus from Rhododendron. Fourier transform infrared (FTIR) spectroscopy was used for obtaining vibrational spectra of 46 petals from Rhododendron. Very minor differences were observed in the FTIR spectra among four subgenuses. For the purpose of rapid differentiation, libraries of spectra were created using samples from each subgenus variety. Spectra of unknown samples were recorded and compared with those of the libraries and the rate of affinity (the match value) was measured automatically using the appropriate software (OMNIC). The results showed that petal samples from different subgenus varieties can be differentiated from each other. The study demonstrates that combining FTIR spectroscopy with appropriate analysis method can classify Rhododendron plants at subgenus level. It offers a potential method for the taxonomic research on plants system.

[Detecting int I 1 gene expression of Pseudomonas aeruginosa with fluorescent quantitative PCR assay].

OBJECTIVE: In order to understand the role of integron, fluorescent quantitative polymerase chain reaction (FQ-PCR)was developed to measure the changes in int I 1 gene expression of Pseudomonas Aeruginosa in biofilm and planktonic cells. METHODS: Three clinical strains of P. aeruginosa with int I 1 gene (SW07, R07 and TH12) were cultured in planktonic cells and biofilm cells. The total RNA of these cultured bacteria were extracted by the conventional method. The FQ-PCR was developed to measure the changes in int I 1 mRNA expression of the P. aeruginosa with bacterial 16s rRNA as an internal control. RESULTS: The three clinical strains of P. aeruginosa expressed int I 1 mRNA in both biofilm and planktonic cells, but with different levels. The int 1 mRNA expressed by the RO7, SW07 and TH12 strains in the biofilm cells were 1.4, 5.7 and 128 times higher than in the planktonic cells, respectively. CONCLUSION: The int I 1 gene expression of P. aeruginosa in the biofilm is up-regulated at mRNA level. The integron may capture and accumulate drug resistance gene cassettes more effectively in the biofilm condition.

[Study on the effect and mechanism of puerarin on the size of infarction in patients with acute myocardial infarction].

OBJECTIVE: To observe the effect of puerarin on infarction size, fatty acids metabolism, inflammatory response and atherosclerotic plaque stability in patients with acute myocardial infarction (AMI). METHODS: Sixty-one patients with AMI were randomly divided into two groups, the control group (n = 30) and the treated group (n = 31). All were treated with conventional treatment, but to the treated group, puerarin injection was given additionally by injecting 500 mg per day for 2 weeks. Before and after treatment, blood levels of free fatty acids (FFA), matrix metalloproteinases-9 (MMP-9) and C-reactive protein (CRP) were assayed, and the size of infarction was determined by Ideker QRS scoring method. RESULTS: Before treatment, the size of infarction was positively correlated to the levels of FFA, MMP-9 and CRP (r = 0.43, 0.42 and 0.39, respectively, all P<0.01). As compared with those before treatment, after treatment, the three parameters lowered by 30%, 41% and 23%, respectively and the size of infarction significantly reduced in the treated group (P<0.01), while in the control group, no significant change was found (P>0.05). CONCLUSION: Puerarin treatment could significantly reduce the size of infarction in patients with AMI, the mechanism is possibly related with its effects in lowering plasma levels of FFA, inhibiting inflammation and stabilizing atherosclerotic plaque.